Many valves in, for example, high and ultra high vacuum systems are of a push-pull type which are manipulated into open and closed positions by hand tools. Some valves of this type also generally do not provide readily available accommodation for automation. Other currently available valves in this art which are automated suffer numerous drawbacks. A primary drawback is in the inability of the previous automation devices to withstand the high temperatures necessarily associated with the operation of ultra high vacuum systems.
In particular, ultra high vacuum systems must be operated at very high temperatures (on the order of about 350 degrees centigrade) for several hours during the initial pumping period. This cleans the vacuum system by what is called "outgassing" the internal surfaces of the system. Thus cleaned, the ultimate desired quality of vacuum can be reached. This is also referred to as the "bake-out" procedure.
Additionally, these high temperatures dictate that valves employed in ultra high vacuum systems be limited as to the materials used in their fabrication. Thus, all the materials in each valve must have inherent characteristics sufficient to withstand the rigors of cycling between normal operating temperatures (room temperature) and approximately 350 degrees centigrade. This usually means stainless steels are used for nearly every part of such a valve. An exception is the usually copper sealing portion of the valve element which is forced onto and against a knife-edge valve seat of stainless steel. Again, because of the temperature extremes, other materials such as rubbers, plastics, aluminum and brass cannot be used at the seal interface. Background materials which describe vacuum systems in more detail particularly regarding the bake-out and outgassing procedures as well as choices of materials include: VARIAN Vacuum Products, Inc. 1993/1994 Catalog, 172; J. F. O'Hanlon, A User's Guide to Vacuum Technology, Chapters 6 and 11 (J. Wiley & Sons 1980); R. R. LaPelle, Practical Vacuum Systems, 18-21 (McGraw-Hill 1972); P. A. Redhead, et. al., The Physical Basis of Ultrahigh Vacuum, 100-105 (Chapman and Hall 1968).
Moreover, a still further consideration involving the high temperature baking out of the ultra high vacuum system is that the cleanliness of the system, in addition to the precision used in manufacturing the valve parts, is what governs the force necessary to achieve and maintain a seal between the copper portion of the valve element and the knife-edge valve seat. A cleaner, more precisely manufactured valve will require less force to maintain valve closure regardless the vacuum forces in the system.
Examples of valves of this type used in ultra high vacuum systems include those made by VARIAN Vacuum Products, Inc., model no. 951-5014, inter alia. Note, this and similar models of valves are currently manually operated by the turning of a drive screw which ultimately pushes in the valve stem to close the valve and pulls it out to open it.
In view of the foregoing, there appears to be a need for valve actuators for existing push-pull valves such that the actuators provide ready, non-manual operation and also endure the extreme conditions inherent in high and ultra high vacuum systems.